The Family Insurance: Kin Selection and Cooperative Breeding in a Solitary Primate (Microcebusmurinus)

Despite substantial physiological costs of lactation, allonursing is common in mammals that share roosts. Hypotheses to explain allonursing among plural breeders include misdirected parental care, milk evacuation, brood parasitism, reciprocity, and kin selection. The necessary behavioral data, in combination with data on kinship and kin recognition, have rarely been available to distinguish among these explanations, however. Here we provide evidence for cooperative nursing and adoption by females in a nocturnal primate, the gray mouse lemur, in which females forage solitarily at night, but form day-time sleeping groups with other females. We observed 34 females in Kirindy Forest, Madagascar, over three consecutive annual breeding seasons and determined genetic relationships among all 170 members of this population. We filmed five breeding groups inside their roosts. Across years, the composition of sleeping groups changed substantially, but they always consisted exclusively of close kin (0.125).